A developer used for developing an electrostatic latent image formed on a latent image carrying member is classified into one for a method using a dry toner, in which a toner containing a colorant, such as a pigment, and a binder resin in a dry state, and another for a method using a liquid developer containing an insulating carrier liquid and a toner dispersed therein.
The method using a dry toner is advantageous in handleability since only a toner in a solid state is handled, but involves problems in contamination due to scattering of a toner and in uniformity upon dispersing a toner.
In the method using a liquid developer containing an insulating carrier liquid and a toner dispersed therein, on the other hand, fine toner particles can be used, and therefore, it is excellent in reproducibility of a thin line image, gradation and color. Furthermore, it is also excellent as a high-speed image forming method.
A liquid developer contains an insulating carrier liquid having dispersed therein a toner, which is formed by kneading a binder resin with a colorant, such as a pigment, and a charge controlling agent for improving the charge characteristics of the toner.
While various kinds of liquids have been known as the insulating carrier liquid for the liquid developer, an isoparaffin has been known as a carrier liquid that is safe and good in handleability, and the use of a silicone oil has been proposed as a carrier liquid that can be applied to wide range of photoreceptors (as described, for example, in Japanese Patent Application Laid-Open No. 43749/1991).
The liquid developer involves such a problem in that the charge controlling agent present in the vicinity of the surface of the toner particles is liable to be dissolved in the carrier liquid to reduce the function of the charge controlling agent.
In the process for forming the toner particles, furthermore, the charge controlling agent may sublime or suffer oxidative decomposition upon processing the kneaded binder resin under heat or upon drying the particles under heat or reduced pressure after forming the particles in the liquid. As a result, the electrostatic charge of the toner is liable to be ununiform to cause problems in fogging and transferring failure due to inverse charge.